Vibrations of the shaft assembly of a machine, in particular of a turbo generator assembly, may be of a variety of types. They may be bending vibrations or transverse vibrations which occur in the direction perpendicular to the shaft. Furthermore, so-called oscillations may occur, i.e. generally changes in the rotational frequency of the shaft, which are damped quasi-periodically, about the system frequency of generally 50 Hz (or 60 Hz), which are generated by, for example, sudden disruptions to the system. A third type of vibrations of the shaft is the actual torsional vibrations which may be generated, for example, by sudden increases in the load on the system. Torsional vibrations are in this case vibrations which are manifested in a non-uniform rotational frequency along the shaft (phase shifts or frequency shifts), i.e. bring about torsion in the shaft.
Torsional vibrations are very small vibrations, generally in the region of 0.01 degrees phase amplitude, which can, however, lead to a very high load on the shaft, and, in particular, breakdown of the natural frequency of such a torsional vibration with the stimulus exciting this torsional vibration may lead to a hazardous escalation of such torsional vibrations, which may even result in breakage of the shaft. In this case, torsional vibrations may have frequencies of a few Hz to 200 Hz, the frequency naturally depending on the material composition and thickness of the shaft, the masses associated with the shaft, and the size of the system. In particular in the case of large systems having long shafts, these torsional vibrations may be extremely critical.
In accordance with the importance of this problem, a large number of patent specifications have already dealt with this topic. For example, U.S. Pat. No. 3,934,459 describes a measuring device and a method for measuring the torsional vibrations of the entire shaft assembly of a turbine generator system. For this purpose, the torsional vibrations are sensed at one or more points on the shaft by means of one or more sensors which are not specified in any more detail. The further-processing of the torsion signals is the main topic of this specification. The further-processing takes place by filtering using bandpass filters and multipliers in order to determine, in the end, the maximum torques.
In addition, mention will be made of U.S. Pat. Nos. 3,885,420, 4,148,222, 4,137,780 and 4,317,371, which all describe measuring devices or methods for measuring torsional vibrations of such electrical systems. For the purpose of detecting the torsional vibrations, toothed wheels connected to the shaft are used as the signal transmitters which produce electrical signals via sensors. The signals are further-processed by means of bandpass filters and multipliers etc. U.S. Pat. No. 4,317,371 describes a special demodulation method for measuring phase shifts. The method comprises the production of lower intermediate frequencies and the subsequent frequency demodulation, as is known per se from radio technology. Another method (U.S. Pat. No. 3,885,420) uses a phase-locked loop (PLL) for demodulation. In addition, reference is made to U.S. Pat. No. 4,444,064, which describes a method in which initially a magnetic pattern is impressed in the shaft, and is subsequently used as the pulse generator. Another concept for measuring torsional vibrations is based on the fact that the voltage at the phase winding clamps of the permanent magnet generator, which is coupled to the shaft, is used for the purpose of determining such vibrations. Evaluation of the generated voltages will give an indication of the torsional vibrations. The evaluation takes place, in turn, by means of frequency demodulation (PLL technique). One advantage of this arrangement is the lower costs in comparison to the toothed wheel solution.